As the result of the declining availability of oil, more emphasis has been directed toward the problem of more effective utilization of coal. Two methods are generally used for removing coal from the ground, either strip mining, in which the coal is merely dug out of the ground by mechanical or hydraulic means and transferred to the place of use, or underground mining using methods such as slurry mining, room and pillar, or long wall.
Comminution of coal into pieces of manageable size has been accomplished by mechanical means, explosives or by chemical means.
Processes for chemical comminution of coal, both above ground and below ground have been disclosed in U.S. Pat. Nos. 3,815,826 to Aldrich et al., 3,870,237 to Aldrich and 4,032,193 to Drinkard et al. According to these processes, the interlayer forces at natural interfaces present in the coal is weakened by contact with a number of reagents such as gaseous anhydrous ammonia, liquid anhydrous ammonia, aqueous ammonia, organic solvents, alcohols containing sodium hydroxide, and aqueous solutions of sodium hydroxide.
Underground gasification of coal has been carried out in a number of cases to extract the energy of the coal while it is still underground. In underground coal gasification processes, a combustible gas is produced which is brought to the earth's surface and transported by pipelines. One difficulty of underground gasification is the low permeability of coal to the flow of gas therethrough. Combustion in the coal seam cannot be carried out efficiently unless an oxygen-containing gas can be passed through the seam. To cope with this problem, it has been the practice to introduce explosives into the coal seam to fracture the coal, or pneuamtic and hydraulic fracturing can sometimes be utilized. Also, permeability can be increased by injecting solvents into the coal seam as taught by U.S. Pat. No. 4,130,164 to Datta. This patent teaches the use of solvents that include various forms of ammonia and methanol that increase the permeability of the coal to the flow of fluid therethrough.
It has now been discovered that by contacting the coal formation with a dry gas such as air, oxygen, oxygen-enriched air, carbon dioxide, argon, nitrogen, methane or helium, permeability of the coal formation is increased to permit fluid flow in in-situ coal gasification processes. Also, gas-drying the coal in combination with mechanical or chemical comminution enhances disintegration of the coal and enables the coal to be more easily removed mechanically or by slurry mining.